Pt900 platinum
Pt900 marks a platinum alloy that is 900 parts in 1,000 pure. Platinum is a white, lustrous metal so chemically stable that, like gold, it dissolves only in aqua regia, and it is widely used for jewellery. In our handling, Pt900 is one of the most familiar marks inside a Japanese ring.
Definition
Under the international standard ISO 9202 and the Japanese Industrial Standard JIS H 6309, the purity of platinum jewellery alloys is given in parts per thousand, in three grades: Pt950, Pt900 and Pt850. In Japan, Pt999 is generally added as a fourth grade for jewellery, and the Japan Mint's hallmarking uses the same four grades. Under ISO and the Japan Jewellery Association, only alloys of Pt850 and above may be called platinum jewellery.
Platinum is not white gold
Because the Japanese word for platinum, hakkin, is written with characters that read as white gold, the two are often confused. They are different metals: white gold is a gold alloy whitened with metals such as palladium, silver or nickel, and it is marked as gold, for example K18WG; platinum is a separate element and is marked Pt. In our handling, a ring stamped K18WG is gold, however white it looks.
Origin
The Spanish called platinum platina del Pinto, little silver of the Pinto river, the origin of its name. The Japan Mint began hallmarking precious metals in 1929, to counter dealers selling fakes, and fake platinum marks still occur: the Mint has considered criminal complaints over silver rings stamped as platinum.
Key facts
| Pt900 | 90 per cent platinum |
|---|---|
| Grades | Pt999, Pt950, Pt900, Pt850 in Japan |
| Standards | ISO 9202, JIS H 6309 |
| Platinum jewellery | Pt850 and above |
| Mint hallmarking | From 1929 |
How to identify it
- The stamp. Pt900, Pt950 or Pt850, often with a maker's mark and, on rings, the stone weights.
- The Mint hallmark. An official mark beside the purity; our guide to Japanese gold and platinum hallmarks explains it.
- Weight. In our handling, platinum feels noticeably heavier than gold of the same size.
- Two-metal pieces. Combined marks such as Pt900 and K18 show where each metal is used.
Where it appears in the collection
Examples include a Tasaki Paraiba tourmaline ring in Pt900 and a Tasaki ruby and diamond ring in Pt900, on the jewellery shelf. Our guide to the Japanese jewellery houses introduces Tasaki and others.
Related entries
- K18 gold: the Japanese gold mark
- Sterling silver 925: the silver standard
Sources
- Platinum, Japanese-language encyclopaedia entry (our translation), for its properties, the name, white gold, ISO 9202 and JIS H 6309, the grades, Pt999, the Mint and the Japan Jewellery Association. ja.wikipedia.org
- Hallmark, Japanese-language encyclopaedia entry (our translation), for the Japan Mint's hallmarking from 1929 and the fake platinum case. ja.wikipedia.org
Last reviewed 15 November 2026. The Japonista Curation and Concierge Team, Tokyo, Japan.